1. Field of the Invention
The present invention relates to a liquid crystal display device, especially to an active matrix type liquid crystal display device, a substrate therefor and processes for production of such devices and substrate.
2. Related Background Art
Recently, display images which have higher resolution have been needed in liquid crystal display devices.
Especially, the active matrix type display panel, which uses thin film switching elements to drive pixels has been rapidly developed because it has been easier to make progress in the development of higher pixel density and higher gradation expansion for the active matrix type liquid display compared to other types of liquid crystal display.
As to the thin film switching elements used for active matrix type liquid crystal display panels, in general, in large panels, which are 12.5 cm (5 inches) or larger (across each diagonal), mainly thin film transistors which use amorphous silicon (a-Si) have been applied, whereas in small panels, which are less than 12.5 cm (5 inches), mainly thin film transistors which use poly-crystalline silicon (p-Si) have been applied.
A schematic view of a liquid crystal display panel using p-Si TFT, is shown in FIG. 12. In FIG. 12, a vertical shift register 303 and a horizontal shift register 304 are connected to a panel display circuit 305 which has p-Si TFT, as the switching elements arranged in a matrix. TV signals transmitted from a video signal circuit 301 pass through the vertical shift register 303 and the horizontal shift register 304 and are applied to pixels in the display circuit 305. 302 is a synchronizing circuit to synchronize the two shift registers 303 and 304. Recently, shift registers 303 and 304 have been made using p-Si and these shift registers have been integrated in the display panel. A cross sectional view of a p-Si TFT is shown in FIG. 13. In FIG. 13, a source region and a drain region, comprised of for example n.sup.+ diffused layer 1403 and n.sup.- diffused layer 1407, are formed in a thin film of poly-silicon on a quartz or glass substrate 1401. Applying voltage to a gate electrode 1406 isolated by a gate insulator film 1405 controls On/Off operation. The n.sup.- diffused layer 1407 is provided especially to reduce electric field under the gate electrode 1406 in the vicinity of the drain, is effective to reduce leakage current between the drain and the source and to increase withstand voltage. 1408 are source and drain electrodes, comprised for example of aluminium. 1410 is an interlayer insulating film comprised of, for example, silicon oxide. 1409 is a surface protective layer comprised for example of silicon nitride film. An equivalent circuit of the liquid crystal panel is shown in FIG. 14. In FIG. 16 pixel electrodes 406 are provided corresponding to points of intersection between a plurality of signal lines 401a-401d and a plurality of scanning lines 402a-402d. The drain of a TFT 403 is connected to each respective pixel electrode. The signal lines 401a-401d are connected to sources of the TFT 403 and the scanning lines 402a-402d are connected respectively to drains of the TFT 403. Video signals from the signal lines 401a-401d are applied to the pixel electrodes 406. Drains of the TFT 403 are connected to storage capacitor 404 in order to store applied electron charge for a long time and another terminal 405 of the capacitor electrode is connected to the same voltage as all pixels or some pixels in the same row.
On the other hand, as to efficiency which is needed for each circuit, in consideration of high definition television, if the frame frequency is 60 Hz, the number of scanning lines is approximately 1000, the horizontal scanning period is approximately 30 .mu.sec (effective scanning period is 27 .mu.sec), and the number of pixels in the horizontal direction is approximately 1500. Television signals are transferred to a buffer at a frequency of approximately 45 MHz. Therefore as to efficiency which is needed for each circuit, especially driving efficiency of the horizontal shift register, an operation frequency of 45 MHz or more is needed.
As will be realized easily from the above explanation, pixel switching elements which have a relatively low driving efficiency are acceptable, but horizontal shift registers having high driving efficiency are needed. Therefore nowadays in liquid crystal display panels, the pixel switching elements or the vertical shift register or both are formed monolithically using TFTs formed in poly-crystalline silicon or amorphous silicon deposited on glass substrates. Other peripheral circuits are formed by attaching IC chips. It has been known to form the peripheral circuit monolithically using poly-crystalline silicon TFTs, but in this case in the circuit the size of transistors is larger because the driving efficiency of each transistor is low.
On the other hand in the case of view-finders used for VTR cameras and reflective type liquid crystal displays, it is important that the substrate is transparent in the visible light region.
To solve these problems, a new proposal has been disclosed in Japanese patent application JP-A-08-069015. The new proposal is as follows. A peripheral circuit portion which constitutes a horizontal shift register is comprised of single crystalline elements and a thin film transistor portion is formed with non-single crystalline semiconductor elements. A portion of the monocrystalline semiconductor region is removed from backside to leave a transparent film under the non-single crystalline semiconductor region.
However in the above mentioned prior art there is the following problem to solve. In JP-A-08-069015 an example that a field oxidation film as an isolation region in a peripheral circuit and an oxide film under thin film transistors are formed in the same thickness by the same process has been disclosed. If the isolation region of the peripheral region in the example is to be smaller, the oxide film under the thin film transistors is to be thinner and the reliability of removing the monocrystalline semiconductor region from the backside is reduced. If the oxide film under the thin film transistors is to be thicker there is a trade-off that the isolation region of the peripheral circuit is to be larger. Therefore it is difficult to satisfy both requirements of reducing cost and upgrading reliability. If the oxide film under the thin film transistors is not used as a stopper in the case of removing the monocrystalline semiconductor region from the backside and another film is used as a stopper, on the contrary to the aforementioned example it is unnecessary that the oxide film under the thin film transistors is as thick as the field oxide film of the isolation region of the peripheral circuit. If the oxide film under the thin film transistors is formed at the same time as forming the field oxide film, an oxide film which is formed after patterning is used as the oxide film under the thin film transistors. Therefore the oxide film which is formed after the patterning is inferior to an oxide film which is oxidized before patterning due to defects such as pinholes and the like. It becomes a factor of not doing well as to the process of the removing the monocrystalline semiconductor region from backside and gives rise to deterioration of picture quality based on point defect or line defect etc. and yield ratio.